Fused metal nanostructured networks, fusing solutions with reducing agents and methods for forming metal networks

ABSTRACT

Reduction/oxidation reagents have been found to be effective to chemically cure a sparse metal nanowire film into a fused metal nanostructured network through evidently a ripening type process. The resulting fused network can provide desirable low sheet resistances while maintaining good optical transparency. The transparent conductive films can be effectively applied as a single conductive ink or through sequential forming of a metal nanowire film with the subsequent addition of a fusing agent. The fused metal nanowire films can be effectively patterned, and the patterned films can be useful in devices, such as touch sensors.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/777,802, to Virkar et al., filed on Feb. 26, 2013, entitled“FUSED METAL NANOSTRUCTURED NETWORKS, FUSING SOLUTIONS WITH REDUCINGAGENTS AND METHODS FOR FORMING METAL NETWORKS,” which is incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to fused nanostructured metal networks. Inaddition, the invention relates to fusing processes using oxidationand/or reducing agents to fuse metal nanowires into a nanostructurednetwork. The fused nanostructures metal networks can be effectively usedfor the formation of transparent conductive films.

BACKGROUND OF THE INVENTION

Functional films can provide important functions in a range of contexts.For example, electrically conductive films can be important for thedissipation of static electricity when static can be undesirable ordangerous. Optical films can be used to provide various functions, suchas polarization, anti-reflection, phase shifting, brightness enhancementor other functions. High quality displays can comprise one or moreoptical coatings.

Transparent conductors can be used for several optoelectronicapplications including, for example, touch-screens, liquid crystaldisplays (LCD), flat panel display, organic light emitting diode (OLED),solar cells and smart windows. Historically, indium tin oxide (ITO) hasbeen the material of choice due to its relatively high transparency athigh conductivities. There are however several shortcomings with ITO.For example, ITO is a brittle ceramic which needs to be deposited usingsputtering, a fabrication process that involves high temperatures andvacuum and therefore is relatively slow and not cost effective.Additionally, ITO is known to crack easily on flexible substrates.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a method for forming afused metal nanostructured network, the method comprising contactingmetal nanowires with a fusing solution. In some embodiments, the fusingsolution comprises a reducing agent source and a metal ion source. Thecontacting step is effective to reduce metal ions to its correspondingmetal element to fuse the metal nanowires together to form the fusedmetal nanostructured network.

In other aspects, the invention pertains to a method for forming a fuseda fused metal nanostructured network, the method comprising irradiatinga metal nanowire film on a substrate surface to form the fused metalnanostructured network.

In a further aspect, the invention pertains to a fusing solution forfusing metal nanowires into an electrically conductive fused metalnanostructured network in which the fusing solution comprises metalnanowires, a metal ion source, and a reducing agent and/or oxidizingagent.

In another aspect, the invention pertains to a fused metalnanostructured network comprising fused metal nanowire segments formingan electrically conductive network substantially free of halides. Insome embodiments, the fused metal nanowire segments comprises a firstmetallic composition, which are fused with a second metallic compositionthat is either the same or distinct from the first metallic compositionto form the fused metal nanostructured network.

Furthermore, the invention pertains to a set of solutions for sequentialapplication comprising a nanowire ink comprising a dispersion of metalnanowires, and a fusing solution. The fusing solution can comprise ametal ion source, and a reducing agent and/or an oxidizing agent,wherein the fusing solution is effective to fuse a film of the metalnanowires upon drying.

In additional embodiments, the invention pertains to a patternedtransparent conductive material comprising a substrate, a fused metalnanostructured network covering a portion of a surface of the substrate,and regions of the surface of the substrate substantially free of metalnanowires and fused metal networks. The fused metal nanostructurednetworks can form an electrically conductive pattern. In someembodiments, the transparent conductive material has a totaltransmission of visible light of at least about 91%. With respect to thepatterned transparent conductive material, a touch sensor can comprise afirst electrode structure and a second electrode structure spaced apartin a natural configuration from the first electrode structure, the firstelectrode structure comprising a first transparent conductive electrodeon a first substrate wherein the first transparent conductive electrodecomprises the patterned transparent conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram showing sintered network along thesubstrate surface forming a conductive pattern with a single pathway.

FIG. 1B is a schematic diagram showing sintered network along thesubstrate surface forming a conductive pattern with a plurality ofelectrically conductive pathways.

FIG. 2A is a schematic diagram showing a capacitance based touch sensor.

FIG. 2B is a schematic diagram showing a resistance based touch sensor.

FIG. 3 is a logarithmic scale plot of resistance data of films 1-10 fromexample 1.

FIG. 3a is a composite of SEM images, taken at different magnifications,of the surface of an AgNW film representative of film sample 5 inExample 1.

FIG. 4 is a logarithmic scale plot of resistance data of films fromexample 2 before and after the application of sintering solution.

FIG. 4a is an SEM image of the surface of an AgNW film representative offilms 1 and 2 in Example 3.

FIG. 5 is an atomic force microscopy image of AgNW network formed withsample Pd3 of Example 7.

FIG. 5a is a composite of SEM images, taken at different magnifications,of the surface of an AgNW film representative of sample Pd1 in Example7.

FIG. 6 is an atomic force microscopy image of AgNW network formed withdiluted copper solution of Example 8.

FIG. 6a is a composite of SEM images, taken at different magnifications,of the surface of an AgNW film representative of sample 1 in Table 11.

FIG. 6b is a composite of SEM images, taken at different magnifications,of the surface of an AgNW film representative of the films formed withCT2 in Table 12, however, having a sheet resistance of about 200 Ohm/sq.

FIG. 7 is an atomic force microscopy image of AgNW network formed withbenzoin based solution sintering of Example 12.

DETAILED DESCRIPTION

Fused metal nanostructured networks are described with low electricalresistance and high optical transparency, which can be formed using areduction-oxidation balanced system that is found to drive fused networkformation. Moreover, fusing can be driven by ripening processes wherematerial can undergo a net transfer of material between locations tolower the free energy. In some embodiments, the fused joints of thenanostructure may comprise a different elemental metal than the metalelement(s) of the metal nanowire components incorporated into the fusedstructure. In particular, fusing agent solutions can be formed with ablend of an acid and reducing agents selected to reduce metal ions insolution to form the fused joints of the nanostructure. Specific metalions can be introduced for reduction to elemental metal to form thefused metal nanostructured network. In additional or alternativeembodiments, oxidizing agents, e.g., appropriate acidic solutions, canbe provided to liberate metal ions from metal nanowire components aseffectively the only metal ion source so that the metal nanowires canthemselves supply metal ions that are reduced to fuse joints within theresulting nanostructure. The discovery that chemical reducing agents canform fused junctions of metal networks has provided the ability to formnew structures as well as to provide alternative approaches for metalnetwork formation in addition to previously discovered halide ion fusingmechanisms. Based on the use of a reducing agent as a fusing agent,nanostructured metal networks can be formed in which the metal at thejoints can be a different metal element composition than the metalelements of the component metal nanowires used to form the structure,and these hybrid structures may be desirable in some applications.

Methods are described to effectively process metal nanowire networkswith selected structures and appropriate reducing agents as well asacids, which can be an oxidizing agent. The fusing solution can becombined with a metal nanowire dispersion to enable processing of thefused metal nanostructured networks from a single ink. In additional oralternative embodiments, the fusing solution comprising the reducingagent and/or optionally metal cations can be added to a deposited metalnanowire film. Through the selected deposition of a fusing solution,patterning can be effectively performed in which highly conductiveregions have fused metal networks and regions of low electricalconductivity are located where no fusing has been performed. Due to thegood electrical conductivity and optical transparency, the fused metalnanostructured networks are well suited for the formation of transparentconductive electrodes, such as for appropriate displays, sensors,photovoltaic cells or the like.

To form stable solutions, it is generally desirable to include an acidalong with the reducing agent so that metal ions in the fusing solutiondo not nucleate for particle formation. While there may be ways toprocess the structures if multiple solutions are used without anoxidizing agent, the processing is generally based on a rough balance ofthe reduction-oxidation system. When combined with metal nanowires in afilm, the poised reduction-oxidation (redox) system induces a netmigration of metal to junctions between adjacent nanowires to fuse thejunctions. The discovery of this redox driven mechanism for fusing metalnanowires into a nanostructured network provides for added flexibilityand processing options for the formation of fused metal nanostructurednetworks. The observation of a driving force to form the fused networksis consistent with a similar driving force observed with metal atommobility induced by a metal halide coating over the metal wires.Additionally, it should be noted that the metal ions can be provided inthe fusing solution or can be formed by etching the wires. The mobileions can diffuse and can be reduced at junctions between the nanowiresresulting in a decrease in free energy. The net changes I the materialsresulting in the fused nanostructure is analogous to an Ostwald ripeningprocess in which for the current ripening process reduction—oxidationreactions evidently drive the mobility of the material to provide for arelatively rapid ripening-type process.

Metal nanowires can be formed from a range of metals. For example, theproduction of a range of metal nanowires is described, for example, inpublished U.S. patent application 2010/0078197 to Miyagishima et al.,entitled “Metal Nanowires, Method for Producing the Same, andTransparent Conductor,” incorporated herein by reference. There has beenparticular interest in silver nanowires due to the high electricalconductivity of silver. With respect to the specific production ofsilver nanowires, see for example, published U.S. patent application2009/0242231 to Miyagisima et al., entitled “Silver Nanowire, ProductionMethods Thereof, and Aqueous Dispersion,” and U.S. 2009/0311530 to Hiraiet al., entitled “Silver Nanowire, Production Method Thereof, andAqueous Dispersion,” and U.S. Pat. No. 7,922,787 to Wang et al.,“Methods for the Production of Silver Nanowires,” all three of which areincorporated herein by reference.

While metal nanowires are inherently electrically conducting, the vastmajority of resistance in the silver nanowires based films is believedto due to the junctions between nanowires. Depending on processingconditions and nanowire properties, the sheet resistance of a relativelytransparent nanowire film, as deposited, can be very large, such as inthe giga-ohm range or even higher. Various approaches have been proposedto reduce the electrical resistance of the nanowire films withoutdestroying the optical transparency. As described herein, lowtemperature fusing to form a metal nanostructured network has been foundto be very effective at lowering the electrical resistance whilemaintaining the optical transparency.

A significant advance with respect to achieving electrically conductivefilms based on metal nanowires has been the discovery of a process toform a fused metal network where adjacent sections of the metalnanowires fuse. In particular, it was discovered in previous work thathalide ions can drive the low temperature fusing of metal nanowires toform fused metal nanostructures. Fusing agents comprising halide anionswere introduced in various ways to successfully achieve the fusing witha corresponding dramatic drop in the electric resistance. Specifically,the fusing of metal nanowires with halide anions has been accomplishedwith vapor acid halides as well as with solutions of halide salts oracid halides.

Metal halides along the surface of metal nanowires are believed toincrease the mobility/diffusivity of the metal ions that result infusing of points of contact or near contact between nanowires to formthe fuse network. Evidence suggests that a metal halide shell forms onthe resulting fused nanowire network when the halide fusing agents areused. While not wanting to be limited by theory, it is believed that themetal halide coating on the metal nanowires results in mobilization ofmetal atoms/ions from the nanowires such that the mobilized ionscondense to form joints between nearby nanowires forming thenanostructured network and presumably lowering the free energy whenforming the fused network with a net movement of metal atoms within thenanostructure.

With respect to processing, fusing of metal nanowires with halide anionshas been successfully accomplished using exposure for short periods oftime to acid halide vapors, by the spraying of halide fusing solutionsonto a metal nanowire film and with the formation of a ink comprisingmetal nanowires and a halide fusing agent that results in a fused metalnanostructured network after depositing and drying. The fused metalnanostructured network has an electrical conductivity at least severalorders of magnitude greater than the unfused nanowire films whilemaintaining good optical transparency and low haze. The use of halideanion fusing agents is described in detail in co-pending U.S. patentapplication Ser. No. 13/530,822, filed Jun. 22, 2012, to Virkar et al.,entitled “Metal Nanowire Networks and Transparent Conductive Material,”published as U.S. Patent Application Publication No. 2013/0341074, andU.S. Pat. No. 9,920,207, filed Oct. 30, 2012 to Virkar et al. (“the'1207 patent”), entitled “Metal Nanostructured Networks and TransparentConductive Material,” both of which are incorporated herein byreference. The '207 patent also discusses what can now be recognized asa combined approach in which silver fluoride dissolved in ethanol isapplied as a fusing solution. In the combined solution, the fluorideions can be effective themselves in forming a metal fluoride, e.g.,silver fluoride, shell over metal nanowires to facilitate metalmigration to fuse adjacent nanowires, and the ethanol can reduce silverions in the solution to also fuse adjacent nanowires, as described indetail herein. It is now appreciated that these fusing solutions caninduce both mechanisms for forming the fused metal nanostructurednetwork.

As described herein, it has been discovered that fused nanostructuredmetal networks can be formed through a reduction-oxidation mechanismthat is a seemingly very different mechanism from the halide drivenmechanism previously discovered. Specifically, reducing agents and/oroxidizing have been discovered to provide a driving force to generatethe fusing of metal nanowires using metal ions from solution to formfused metal nanostructured networks. With respect to the use of reducingagents, generally metal ions are in the solution as a source of cationsfor reduction to the elemental metal. The reduced metal is found toeffectively fuse adjacent metal nanowires to form the desired fusedmetal nanostructured network. The metal ions for reduction can beintroduced through the addition of a selected metal salt to the fusingsolution or through the in situ formation of the metal ions from metalnanowire components. Specifically, addition of an oxidizing acid to thefusing solution can etch the metal nanowires to form metal cations ofthe metal nanowire metal. An oxidizing agent can also balance a reducingagent to stabilize a fusing solution and/or drive the fusing reactionunder more controlled conditions. Moreover, the fusing solution mayprovide or produce metal ions which can be reduced at intersections orclose contact points between wires and fuse the structures together.

Fusing solutions generally comprise a reducing agent that induces thefusing and may further comprise a metal ion source and/or metalnanowires. Acids can be useful in the fusing solutions to help dissolveor partially remove polymer coatings on the nanowires from the nanowiresynthesis process and an oxidizing acid can be useful to buffer thepotential to control the deposition process and stabilize the fusingsolution. In particular, it is generally desirable to include the metalion source in the fusing solution so that a separate solution with themetal ion source is not separately deposited. However, with respect tothe metal nanowires, it may or may not be desirable to first deposit themetal nanowires into a film and then perform the fusing process throughthe addition of the fusing solution. If a single solution is used withthe metal nanowires and the fusing agents, it may be desirable to formthe solution shortly prior to use if the metal nanowires are etched bythe other components of the fusing solution. As noted above, thereducing agent may or may not be a solvent, which would becorrespondingly present at high concentrations. Metal sources can be ametal salt dissolved into the solution or an oxidizing agent thatgenerates metal ions from the metal nanowires.

Regardless of the source of the metal ions in solution, the reducingagent can be effective to deposit metal at junction locations ofadjacent metal nanowires to fuse adjacent metal nanowires to form thefused nanostructured network. Moreover, once metal ions are present,they can diffuse to areas between nanowires and can be reduced atjunctions between nanowires, which result in an electrochemicalOstwald-type ripening since deposition at the junction points can bethermodynamically more favorable than deposition along the wiresegments. A metal salt added to the fusing solution can comprise thesame metal element or a different metal element relative to the initialmetal nanowires added to the network. In this way, the joints of thefused nanostructured network can comprise the same or different metalsof the initial metal nanowire components incorporated into the network.As described further below, in some embodiments, it can be desirable forthe joints to be formed from a different metal than the metal nanowirecomponents that are fused into the nanostructure.

A suitable reducing agent should be able to drive the reduction of ametal ion to its elemental form: M^(a+)→M⁰, where M is the selectedmetal, “a” is the oxidation state of the metal cation and M⁰ indicatesthe elemental form of the metal. It has been found that a mild reducingagent, such as certain organic compounds, can be sufficient to drive thefusing process. For example, an alcohol solvent, such as ethanol, candrive the fusing for at least some metals. The results herein suggestthat the reduced metal tends to preferentially deposit at junctionpoints of adjacent metal nanowires to facilitate formation of the fusedmetal nano-structured network.

Selected reducing agents can be in high concentrations, for example asthe solvent or a component of a solvent mixture or as a solute at aselected concentration. Various alcohols can be used as suitablereducing agents for silver, palladium, and copper. In particular,ethanol and propylene glycol are found to be effective for the reductionof metals to form the fused metal networks. Alcohols can be oxidized toaldehydes/ketones or to carboxylic acids while correspondingly reducingthe metal cations. Alternatively other reducing agents, such as organicor inorganic reducing agents can be added to the fusing solution at anappropriate concentration.

Combined systems can involve a fusing solution comprising metal halidesand a reducing agent. These systems are observed to form metal halideshells over the fused metal nanostructure network. Presumably, thesesystems can have fusing of junctions through one or both mechanisms.

In circumstances in which the metal ions for fusing of the metal networkare supplied from metal nanowire components, the fusing solutiongenerally comprises both an oxidizing agent and a reducing agent. Withrespect to in situ generation of metal cations, an oxidizing acid, suchas nitric acid can be used to etch, i.e., oxidize, the metal nanowiresto generate metal cations. Acids may also be useful to remove anyresidual polymers that may be associated with metal nanowires form theirsynthesis, as obtained in commercial samples. The presence of oxidizingagents and reducing agents within the solution in some sense buffers theredox (reduction-oxidation) potential of the system, and the redoxagents can be balanced to achieve desired results. An excess ofoxidizing agents can etch the metal more than desired, and too strong ofan oxidizing agent may quench the reduction of the metal so that nofusing may occur. If the oxidation agent and the reducing agent arereasonably balanced, metal is etched to supply metal ions into thesolution, and the reducing agent reduces the metal ions to formelemental metal that evidently preferentially accumulates at junctionpoints of adjacent metal nanowires. During the ripening process, metalmigrates gradually from the metal wires to form fused junctions. Thus,it is observed that there is a net metal migration from the metalnanowire segments of the lattice to the joints of the network. While notwanting to be limited by theory, this observation strongly suggests adecrease in free energy through the migration of metal to the jointsfrom the connected segments. The rate of fusing may be influenced by thebalance of the oxidizing and reducing agents. The process can beappropriately quenched following a desired degree of fusing of thejoints of the fused metal network. Quenching can be accomplished, forexample, through drying, rinsing, dilution or a series of processingsteps.

In summary, the effective fusing of metal nanowires based on metalreduction-oxidation mechanisms described herein involves theintroduction of a significant concentration of metal ions in solutionalong with an oxidation agent and/or reducing agent. Each activationagent generally induces a reaction, which is generally reversible, thatresults in metal migration with interconversion from the ionized andelemental, i.e., unionized, forms. The ripening process results in a netmigration to fuse the metal nanostructured network. The metal ions canbe added to the fusing solution, or the metal ions can be generated insitu from metal nanowires themselves. The reducing agent can be thesolvent and/or a reducing agent added to the fusing solution. Withinthis conceptual framework a range of processing approaches can beadapted successfully. In addition, the fusing mechanisms can be used toconveniently pattern the fused metal nanostructured network along asubstrate surface. The fusing achieved with reduction can be used toeffectively form conductive films with good transparency and low haze.

As noted above, metal ions for forming fused junctions can be generatedfrom the nanowire starting components themselves or the metal ions canbe added to the solution. The metal ions in solution can comprise thesame element as the metal nanowires to be fused into the network so thatthe fused junctions of the network have the same metal as the nanowires.However, through the addition of the metal ions to the solution, thedeposited metal can be a different metal element than present in theinitial metal nanowire starting material. It may be desirable to use adifferent metal at the joints forming the fused network to potentiallyreduce effects of electromigration of metal during uses of the fusednetworks as electrically conductive films. The fusing solution cansimilarly comprise a mixture of metal ions, which may or may not bereduced from solution at the same rate. In general, the fused metalnanostructured network can comprise the same or different metals formingfused joints relative to the core metals of the fused network. Metaldeposited from the solution by reduction may coat the nanostructuredlattice in addition to forming joints to fuse the network.

In contrast with respect to the earlier work involving halide ions todrive the fusing of metal nanowires, the fusing driven by a reducingagent generally is not expected to form a core shell structure, so thatthe fused metal nanostructured networks described herein differ in thisrespect from the earlier described fused networks. It has been proposedthat the core—metal halide shell structure can influence the opticalproperties of the metal network due to a lower reflectivity of metalhalides, so the fused metal networks described herein may offer slightlydifferent optical properties. It is believed that a lower metal, e.g.,metal nanowire, loading can be successful to achieve desired electricalconductivities with fusing in comparison with methods based on unfusedmetal nanowire networks. The ability to lower the loading of metal inthe network may allow for better overall optical properties for fusednanostructured metal networks compared to non-fused systems whencompared at the same sheet resistances and for better contrast ofconductivity in patterned systems. Also, as noted above, reduction ofmetal from the fusing solution can involve a different metal elementthan comprising the metal nanowire starting materials so that a metalcomposite structure can be formed with respect to the fused metalnetwork involving a different metal along at the fused joints andpossibly covering other portions of the network. Thus, the fused metalnetworks formed with reducing agents can have various differencesrelative to the fused metal networks formed with halide ions.

However, the fused metal nanostructured networks formed with reducingagents can have good electrical conductivity and good opticaltransparency, which is common with the fused metal networks formed usinghalide ions. The improved fused/sintered metal nanowire networksdescribed herein can achieve simultaneously desirably low sheetresistance values while providing good optical transmission. In someembodiments, the fused metal nanowire networks can have opticaltransmission at 550 nm wavelength light of at least 85% while having asheet resistance of no more than about 100 ohms/square. Opticaltransmission is referenced in the description and claims with thecontribution of the substrate removed, and results provided in theExaminer are explained in the specific examples. In additional oralternative embodiments, the fused metal nanowire networks can haveoptical transmission at 550 nm of at least 90% and a sheet resistance ofno more than about 250 ohms/sq. Based on the ability to simultaneouslyachieve good optical transparency and low sheet resistance, the fusedmetal nanowire films can be used effectively as transparent electrodesfor a range of applications. The loading of the nanowires to form thenetwork can be selected to achieve desired properties.

Processing can involve the use of a single solution or ink in which thefusing agents are combined with the metal nanowires, or the multiplesolutions can be used, which may be deposited sequentially. In someembodiments, the metal nanowires, a metal ion source, and reducing agentcan be combined as desired into two or three separate solutions. The useof multiple solutions can introduce greater control on the process, suchas control of processing times or to provide for patterning. Variousprocessing options are described in detail below. With respect topatterning, the metal nanowires can be formed into a film, and a fusingsolution(s) can be applied to selected locations along the nanowire filmto process the film at the selected locations into a fuse nanostructuresnetwork while leaving other locations as an unfused film. The fusedfilms can have a very significantly lower electrical resistance comparedwith the unfused films such that a pattern is formed of electricallyconductive regions and electrically insulating regions. The patternedfilms can be used for functional devices such as touch sensors or thelike.

The transparent conductive films that are formed from the fusednanostructured metal networks are suitable for various applications. Forexample, some solar cells are designed to have an electrode along thelight receiving surface, and a transparent conductive electrode may bedesirable along this surface. Also, some display devices can be madewith a transparent conductive electrode. In particular, touch inputs canbe effectively formed with the transparent conductive films describedherein, and the efficient patterning of the fused nanowire films can beused to form corresponding patterned touch sensors. As described furtherbelow, touch inputs or sensors generally operate based on change ofcapacitance or a change of electrical resistance upon touching of thesensor surface. Thus, the processing approaches described herein canprovide significant commercial applicability for the formation oftransparent conductive films.

The discovery that fused nanostructured metal networks can provide highcontrast in electrical conductivity relative to unfused counterpartmetal nanowire films, has provided a powerful tool for patterning ofthese films through patterned addition of the fusing agents. Thisdiscovery related to fused networks allows for the opportunity toprovide similar patterning functions through other approaches for fusingthe nanowires. In particular, radiation based fusing can be performedusing patterned radiation delivery. The radiation can be selected toperform desired fusing of the nanowires without excessive damage to thesubstrate. In particular, infrared light can be a desirable radiationsource for fusing the nanowires due to reasonable absorption by metaland low absorption by polymer substrates. Thus, the patterningapproaches can be broadened to include halide based fusing agents,reduction-oxidation based fusing agents and radiation, which can each beeffective to provide patterns with high contrast with respect toelectrical conductivity between fused and unfused regions and with lowvisible differences between electrically conductive and non-conductiveregions.

Metal Nanowires and Fusing Solutions

Three components, metal nanowires, a reducing agent and a metal ionsource, are brought together to form the fused nanostructured metalnetworks in a film. In general, fusing solutions generally also comprisean acid in an appropriate concentration to stabilize the solutions,moderate the reaction and in some embodiments to etch the metalnanowires to supply metal ions. The film components are delivered to aselected substrate surface. The components can be delivered in a singleor multiple solutions for application to the substrate surface. Inparticular, it can be desirable to form a nanowire film to which afusing solution is added. Suitable metal ion sources can comprise metalsalts that directly provide a desired metal ion for reduction to thecorresponding metal or an oxidizing agent to oxidize metal from themetal nanowires as a source of metal ions for fusing the network. If ametal salt is provided, the metal ion may or may not involve the samethe metal element as the metal nanowires.

In general, the nanowires can be formed from a range of metals, such assilver, gold, indium, tin, iron, cobalt, platinum, palladium, nickel,cobalt, titanium, copper and alloys thereof are desirable due to highelectrical conductivity. Commercial metal nanowires are available fromBlue Nano (North Carolina, U.S.A.), Seashell Technologies (California,U.S.A.) and KeChuang Advanced Materials (China). Silver in particularprovides excellent electrical conductivity, and commercial silvernanowires are available. To have good transparency and low haze, it isdesirable for the nanowires to have a small range of diameters. Inparticular, it is desirable for the metal nanowires to have an averagediameter of no more than about 250 nm, in further embodiments no morethan about 150 nm, and in other embodiments from about 10 nm to about120 nm. With respect to average length, nanowires with a longer lengthare expected to provide better electrical conductivity within a network.In general, the metal nanowires can have an average length of at least amicron, in further embodiments, at least 2.5 microns and in otherembodiments from about 5 microns to about 100 microns, although improvedsynthesis techniques developed in the future may make longer nanowirespossible. An aspect ratio can be specified as the ratio of the averagelength divided by the average diameter, and in some embodiments, thenanowires can have an aspect ratio of at least about 25, in furtherembodiments from about 50 to about 5000 and in additional embodimentsfrom about 100 to about 2000. A person of ordinary skill in the art willrecognize that additional ranges of nanowire dimensions within theexplicit ranges above are contemplated and are within the presentdisclosure.

The formation of the metal nanowire network comprises the formation of adispersion of the metal nanowires in a suitable liquid and applying thedispersion as a coating onto the selected substrate surface. Theconcentration of the dispersion can be selected to obtain a gooddispersion of the nanowires to provide for a desired degree ofuniformity of the resulting coating. In some embodiments, the coatingsolution can comprise at least about 0.005 wt %, in additionalembodiments from about 0.01 wt % to about 5.0 wt % metal nanowires, insome embodiments from about 0.02 wt % to about 4 wt %, and in furtherembodiments from about 0.05 wt % to about 2.5 wt % metal nanowires. Aperson of ordinary skill in the art will recognize that additionalranges of metal nanowire concentrations within the explicit ranges aboveare contemplated and are within the present disclosure. Similarly, theliquid for forming the dispersion can be selected to achieve gooddispersion of the nanowires. For example, water, aqueous solvents,alcohols, such as ethanol, isopropyl alcohol or isobutyl alcohol, ketonebased solvents, such as methyl ethyl ketone, organic coating solvents,such as toluene or hexane, or the like or mixtures thereof, aregenerally good dispersants for metal nanowires. For embodiments in whichmetal nanowires are deposited to first form a film prior to contactingwith a fusing solution, there can be greater flexibility in selection ofa liquid to disperse the nanowires since the liquid does not need to becompatible with the fusing process or fusing agents.

The fusing solution generally comprises a reducing agent, a metal ionsource, and generally an acid, which can be an oxidizing acid. Thefusing solution can be combined with the metal nanowires to form asingle solution for forming transparent conductive files, or the fusingagent can be applied separately. Various solvents and combination ofsolvents can be used for the fusing solutions described herein. Thetable below provides a list of solvent with properties of the solventsoutlined in detail, and additional solvents include, for example,methanol, butanediol, hydroxylacetone, mixtures thereof, mixtures withthe solvents in the following table and mixtures of solvents listed inthe table.

Polarity Index Boiling (H₂O = 9)/ Surface Point Density Viscosity DipoleTension Name Structure (° C.) (g/cm₃) (cP) Moment (D) (mN/m) Ethanol

78 0.79 1.0 (20° C.) 5.2 22.0 IPA

83 0.79 1.96 (25° C.) 3.9 21.7 1-Butanol

119 0.81 2.94 (20° C.) 4 24.2 2-Butanol

99 0.81 3.7 (25° C.) <4 23.0 Isobutanol

108 0.80 3.95 (20° C.) Propylene Glycol

188 1.04 48.6 (25° C.) 36

In some embodiments, the fusing solution can comprise a metal salt or acombination thereof. In general, the metal ion in the salt can be thesame metal element as the metal element of the nanowires or a differentmetal element. In general, the metal element can be selected as desiredand corresponds with a metal having good electrical conductivity.Suitable metal ions include, for example, ions of silver (Ag⁺), copper(Cu⁺²), gold (Au⁺³), palladium (Pd⁺²), lead (Pb⁺²), aluminum (Al⁺³),nickel (Ni⁺² or Ni⁺³), cobalt (Co⁺² or Co⁺³), zinc (Zn⁺²), iron (Fe⁺² orFe⁺³), tin (Sn⁺² or Sn⁺⁴), or a mixture thereof. In general, the saltscan comprise a halide anion, e.g. (AgF), or have an anion to providedesired solubility or reactivity. Suitable anions can comprise bases ofcarboxylic acids, e.g., acetate, trifluoromethane sulfonate (TMS),heptafluorobutyrate (FHB), and hexafluoroantimonate (HFA), combinationsthereof or the like. The anion can correspond to an oxidizing acid,e.g., nitrate, perchlorate and/or sulfate, to provide desiredfunctionality to the fusing solution.

With respect to metal ions, the fusing solution can comprise metal ionsgenerally from about 0.000001M to about 1M, in further embodiments fromabout 0.00001M to about 0.1M, and in additional embodiments from about0.0001M to about 0.01M. The metal ions can be generated in situ throughthe oxidation of metal nanowires. If the metal nanowires are mixed withthe fusing solution before application to a substrate, the solution canbe combined shortly before use to avoid excessive etching of the metalnanowires. A person of ordinary skill in the art will recognize thatadditional ranges of metal ion concentrations within the explicit rangesabove are contemplated and are within the present disclosure.

The fusing solution can comprise an acid to adjust the acidconcentration or pH, which in some embodiments is evaluated an alcoholsolution. The acid concentration/pH may influence the reductionpotentials, solubilities of reactants, solution stabilities and otherproperties. Generally, the fusing solutions have a pH adjusted throughthe addition of an acid, and the pH can be from about 0.5 to about 6, infurther embodiments from about 1 to about 5.5 and in other embodimentsfrom about 1.5 to about 5. With respect to acid concentrations, an acid,e.g., a strong acid, can be added generally in concentrations at leastabout 0.000001M, in further embodiments from about 0.0000025M to about0.05M and in additional embodiments from about 0.000005M to about 0.01M.The acids may also remove at least polymer, such as polyvinylperolidone(PVP) that may be coating commercial nanowires. Suitable acids caninclude weakly oxidizing acids (i.e., moderate oxidizing activity fromH⁺ ions), such as HCl, phosphoric acid, carboxylic acids, orcombinations thereof. Suitable strong oxidizing acids generally lowerthe pH while providing a significant oxidizing agent based on the anion,which can influence the potentials in the fusing solution and can beused to etch metal nanowires as a metal ion source. Suitable strongoxidizing acids include, for example, HNO₃ (nitric acid), H₂SO₄(sulfuric acid), HClO₄ (perchloric acid), mixtures thereof and the like.A person of ordinary skill in the art will recognize that additionalranges of pH and acid concentrations within the explicit ranges aboveare contemplated and are within the present disclosure.

The reducing agent can be provided as the solvent and/or as an additiveto the solvent. For example, some alcohols can be useful as a reducingagent. For the fusing solutions described herein, suitable alcoholsinclude, for example, methanol, ethanol, isopropanol, iso-butanol,2-butanol, propylene glycol, sugars and mixtures thereof. Ethanol can beoxidized to form acetaldehyde or acetate while reducing a metal ion tothe elemental metal, and other alcohols can be similarly oxidized whenfunctioning as a reducing agent. When a reducing agent is provided asadditive to the solvent, a wide range of organic and inorganic compoundscan be used. In general, the reducing power of the compound can be a nota very strong one, on the basis that a stable fusing solution isdesirable. On the other hand, the reducing agent must be strong enoughto reduce the silver and/or other metal ions to elemental metal underthe condition of the fusing step. Inorganic and organometalliccompounds, typically metal salts and complexes, can be used when theyare soluble in the fusing solution solvent. Useful salts include, forexample, nitrate or sulfate salts and complexes of metal ions such asV²⁺, Fe²⁺, Cr²⁺, Sn²⁺, Ti³⁺, and the like. Other inorganic reducingagents useful for fusing solutions are alkaline metal, ammonium or othersalts of oxidizable anions, such as sulfite, hydrosulfite, thiosulfate,phosphite, hydrogenphosphite, oxalate, or the like or combinationsthereof. Furthermore, nanoparticle suspensions of reducing metal, e.g.,zinc, iron, aluminum, magnesium, and the like, may be used inappropriate amount as reducing agents. The suspensions may furthercontain stabilizing materials, e.g., surfactant or dispersant, to aidthe dispersion of the nanoparticles in solution. Organic reducingagents, in addition to those that also function as solvent, can beparticularly useful in the present invention. Suitable organic reducingagents include but not limited to phenolic compounds, such as phenol,aminophenol, hydroquinone, pyrogallol, catechol, phenidone,4-amino-3-hydroxy-1-naphthalenesulfonic acid, and the like; polyolsincluding sugar alcohols; sugars, such as mono-saccharides anddisaccharides; hydroxylamine and derivatives; aldehydes; □-hydroxycarbonyl compounds such as hydroxyketones like benzoin, furoin,hydroxyacetone; hydrazide derivatives such as phthalhydrazide, adipicacid dihydrazide, phenidone, and the like; reduced aromatic compoundssuch as 1-methyl-1,4-cyclohexadiene, dihydrodiazine, and the like; andcombinations thereof. In general, a reducing agent can be incorporatedinto the fusing solution at a concentration from about 0.001 mM to about1000 mM, in further embodiments from about 0.01 mM to about 100 mM, andin additional embodiments from about 0.1 mM to about 10 mM, and adesired concentration generally is influenced by the chemistry of aselected agent or combination of agents and a person of ordinary skillin the art can evaluate these issue empirically based on the teachingsherein. A person of ordinary skill in the art will recognize thatadditional ranges of reducing agent concentrations within the explicitranges above are contemplated and are within the present disclosure. Ifan organic additive is supplied as a reducing agent, various solventscan be suitable, such as isopropyl alcohol, isobutyl alcohol,formaldehyde, acetone, other ketones, other aldehydes, mixtures thereof,and the like.

Procedures for Forming Fused Metal Network

Fused nanostructured metal films are generally on a selected substratesurface. The formation of the films generally comprises deposition ofprecursor liquid(s) and fusing. A single liquid can be deposited to formthe metal network, or a plurality of liquids can be used, in which onedispersion comprises metal nanowires that can be deposited andsubsequently fused with a fusing agent. Processing with a single liquidgenerally involves deposition of a solution with dispersed metalnanowires, a metal ion source and a reducing agent. Processing with aplurality of liquids generally involves liquids that collectively stillcomprise metal nanowires, a metal ion source and a reducing agent, whichare segregated within the two or more liquids for delivery. The use of asingle liquid provides for a reduction of processing steps, while theuse of a plurality of processing liquids provide some additionalflexibility. For example, if a nanowire film is first deposited, afusing solution can be added to the nanowire film, which can provide forpatterning as described further below, adjustment of relative amounts ofreactants, use of different solvent systems for the different solutions,use of etching acids in fusing solutions that avoids the need to mixsolutions immediately prior to use and other potential options.Processing can be performed at low temperatures, such as at roomtemperature. Processing approaches using the fusing of nanowires asdescribed herein can be effectively adapted to form various devicecomponents, such as touch sensors described below.

Any reasonable coating approach can be used, such as dip coating, spraycoating, knife edge coating, bar coating, Meyer-rod coating, slot-die,gravure, spin coating or the like. After forming the coating with thedispersion, the nanowire network can be dried to remove the liquid. Thedried film of metal nanowires can then be processed to achieve nanowirefusing.

As described in the Examples below, the processing approaches describedherein result in the fusing of the metal nanowires. This fusing isbelieved to contribute to the enhanced electrical conductivity observedand to the improved transparency achievable at low levels of electricalresistance. The fusing is believed to take place at points of nearcontact of adjacent nanowires during processing. Thus, fusing caninvolve end-to-end fusing, side wall to side wall fusing and end to sidewall fusing. The degree of fusing may relate to the processingconditions. As described further below, short processing times arebelieved to contribute good fusing without degradation of the nanowirenetwork.

For the multiple solution approach, a first solution is generallydeposited to form a metal nanowire film. The metal nanowire film may ormay not be dried prior to further processing. A fusing solution orsolutions can then be added to the metal nanowire film to perform thefusing. In some embodiments, it is believed that fusing occurs duringthe drying of the fusing solution in which the drying process increasesmetal ion concentrations. As the material dries, it is believed thatliquid can pool to areas of lower chemical potential in the film betweennanostructures. The films can be dried, for example, with a heat gun, anoven, a thermal lamp or the like. Generally, the films can be heated totemperatures from about 50° C. to about 100° C. during drying. Afterdrying, the films can be washed one or more times, for example, with analcohol or other solvent or solvent blend, such as ethanol or isopropylalcohol, to removed excess solids to lower haze.

For single solution processing, a single coating step provides for thedeposition of metal nanowires and fusing agents together. Generally, asingle solution can be mixed generally no more than about 24 hours priorto use, in some embodiments no more than about 10 hours before use andin further embodiments no more than about 5 hours prior to use. In someembodiments, the solutions can be combined immediately prior to coatingas part of the coating process. After forming the film, the film can bedried, and it is believed that the fusing takes place during the drying.The drying can be performed as described in the previous paragraph inthe context of the separate fusing solution. The fused network can bewashed one or more times, for example, with an alcohol or other solventor solvent blend to removed excess solids.

Electrically Conductive Film Structure and Properties

The conductive films described herein generally comprise a substrate anda fused metal nanowire network deposited on a surface or portion thereofof the substrate. An optional polymer coating can be placed over themetal nanowire network to protect and stabilize the fused nanowirenetwork. The parameters of the metal nanowires can be adjusted toachieve desirable properties for the fused network. For example, ahigher loading of nanowires can result in a lower electrical resistance,but transparency can decrease with a higher nanowire loading. Through abalance of these parameters, desirable levels of electrical conductivityand optical transparency can be achieved. The nanowires in the improvednetworks are fused, as is observed in scanning electron micrographs. Itis believed that the fusing of the nanowires results in the improvedelectrical conductivity while maintaining high levels of opticaltransparency. Having a network with fused nanowires should provide astable electrically conductive structure over a reasonable lifetime of acorresponding product.

In general, suitable substrates can be selected as desired based on theparticular application. Substrate surfaces can comprise, for example,polymers, glass, inorganic semiconductor materials, inorganic dielectricmaterials, polymer glass laminates, composites thereof, or the like.Suitable polymers include, for example, polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyacrylate, poly(methylmethacrylate), polyolefin, polyvinyl chloride, fluoropolymer, polyamide,polyimide, polysulfone, polysiloxane, polyetheretherketone,polynorbornene, polyester, polystyrene, polyurethane, polyvinyl alcohol,polyvinyl acetate, acrylonitrile-butadiene-styrene copolymer,polycarbonate, a copolymer thereof or blend thereof or the like.Furthermore, the material can have a polymer overcoat placed on thefused metal nanowire network, and the overcoat polymers can comprise thepolymers listed for the substrates above. Moreover, other layers can beadded on top or in between the conductive film and substrate to reducereflective losses and improve the overall transmission of the stack.

Following fusing of the metal nanowires into a network, the individualnanowires are no longer present, although the physical properties of thenanowires used to form the network are directly reflected in theproperties of the fused nanostructured network. As noted above theamount of nanowires delivered onto the substrate can involve a balanceof factors to achieve desired amounts of transparency and electricalconductivity. While thickness of the nanowire network can in principlebe evaluated using scanning electron microscopy, the network can berelatively fragile, which can complicate the measurement. In general,the fused metal nanowire network would have an average thickness of nomore than about 5 microns, in further embodiments no more than about 2microns and in other embodiments from about 25 nm to about 500 nm.However, the fused nanowire networks are generally relatively openstructures with significant surface texture on a submicron scale, andonly indirect methods can generally be used to estimate the thickness.The loading levels of the nanowires can provide a useful parameter ofthe network that can be readily evaluated, and the loading valueprovides an alternative parameter related to thickness. Thus, as usedherein, loading levels of nanowires onto the substrate is presented asmicrogram or milligrams of nanowires for a square centimeter ofsubstrate. In general, the nanowire networks can have a loading fromabout 0.01 milligrams (mg)/m² to about 200 mg/m², in further embodimentsfrom about 0.025 mg/m² to about 150 mg/m², and in other embodiments fromabout 0.05 mg/m² to about 100 mg/m². A person of ordinary skill in theart will recognize that additional ranges of thickness and loadingwithin the explicit ranges above are contemplated and are within thepresent disclosure.

Electrical resistance can be expressed as a sheet resistance, which isreported in units of ohms per square (Ω/□ or ohms/sq) to distinguish thevalues from bulk electrical resistance values according to parametersrelated to the measurement process. Sheet resistance of films isgenerally measured using a four point probe measurement or an equivalentprocess. In the Examples below, film sheet resistances were measuredusing a four point probe, or by making a square using a quick dryingsilver paste. The fused metal nanowire networks can have a sheetresistance of no more than about 200 ohms/sq, in further embodiments nomore than about 100 ohms/sq, and in other embodiments no more than about60 ohms/sq. A person of ordinary skill in the art will recognize thatadditional ranges of sheet resistance within the explicit ranges aboveare contemplated and are within the present disclosure. In general,sheet resistance can be reduced by increasing the loading of nanowires,but an increased loading may not be desirable from other perspectives asdescribed further below, and the loading is not as significant asachieving good fusing for improving the sheet resistance.

For applications as transparent conductive films, it is desirable forthe fused metal nanowire networks to maintain good optical transparency.In general, optical transparency is inversely related to the loading,although processing of the network can also significantly affect thetransparency. The optical transparency can be evaluated relative to thetransmitted light through the substrate. For example, the transparencyof the conductive film described herein can be measured by using aUV-Visible spectrophotometer and measuring the total transmissionthrough the conductive film and support substrate. Transmittance is theratio of the transmitted light intensity (I) to the incident lightintensity (I_(o)). The transmittance through the film (T_(film)) can beestimated by dividing the total transmittance (T) measured by thetransmittance through the support substrate (T_(sub)). (T=I/I_(o) andT/T_(sub)=(I/I_(o))/(I_(sub)/I_(o))=I/I_(sub)=T_(film)) Thus, thereported total transmissions have the transmission through the substrateremoved from the values. While it is generally desirable to have goodoptical transparency across the visible spectrum, for convenience,optical transmission can be reported at 550 nm wavelength of light.Alternatively or additionally, transmission can be reported as totaltransmittance from 400 nm to 700 nm wavelength of light, and suchresults are reported in the Examples below. In general, for the fusedmetal nanowire films, the measurements of 550 nm transmittance and totaltransmittance from 400 nm to 700 nm (or just “total transmittance” forconvenience) are not qualitatively different, although in someembodiments of transparent conductive films total transmission may be1-2% higher than 550 nm transmission. In some embodiments, the filmformed by the fused network has a total transmittance of at least 80%,in further embodiments at least about 85% and in additional embodiments,at least about 90%. Transparency of the films on a transparent polymersubstrate can be evaluated using the standard ASTM D1003 (“Standard TestMethod for Haze and Luminous Transmittance of Transparent Plastics”),incorporated herein by reference. As noted above, the correlation ofgood optical transparency with low electrical resistance can beparticularly desirable. In some embodiments with a sheet resistance from20 ohm/sq to about 150 ohm/sq, the films can have a total transmittanceof at least about 86%, in further embodiments at least about 88% and inother embodiments from about 89% to about 95%. In one embodiment, thefilm can have a sheet resistance of no more than about 75 ohm/sq and atotal transmittance of at least about 85%. In another embodiment, thefilm can have a sheet resistance of no more than about 175 ohm/sq and atotal transmittance of at least about 90%. A person or ordinary skill inthe art will recognize that additional ranges of optical transmissionwithin the explicit ranges above are contemplated and are within thepresent disclosure.

The sintered metal networks can also have low haze along with hightransmission of visible light while having desirably low sheetresistance. Haze can be measured using a hazemeter based on ASTM D1003referenced above, and the haze contribution of the substrate can beremoved to provide haze values of the transparent conductive film. Insome embodiments, the sintered network film can have a haze value of nomore than about 0.5%, in further embodiments no more than about 0.45%and in additional embodiments no more than about 0.4%. A person ofordinary skill in the art will recognize that additional ranges of hazewithin the explicit ranges above are contemplated and are within thepresent disclosure.

Patterning

The processing approaches described herein can be used for efficientpatterning of films to form patterns of electrically conductive regionsand less conductive regions with desirable optical transparency acrossthe film. In particular, since the sintering/fusing process is performedchemically, the controlled delivery of the sintering agent to selectedportions of a metal nanowire film can form a sintered metal network atthe portions of a film contacted with the sintering agent, while theremaining portions of the metal nanowire film remain un-sintered. Ofcourse, control of the sintering agent delivery does not have to beperfect for the patterning to be effective for appropriate applications.The patterning based on selective delivery of fusing agents along thesubstrate can be effective to form a pattern that is effectivelyinvisible to the eye under white light. Based on the discovery ofeffective patterning based on selective fusing of a metal nanowire film,similar patterned structures can be formed using radiation based fusingof the metal nanowires in which the radiation is directed in a pattern.While patterning based on fusing the nanowires according to the selectedpattern has many desirable aspects, patterning through the removal ofmaterials or subtractive patterning can also be performed.

A particular pattern of fused conductive network along the substratesurface generally is guided by the desired product. Of course, for someproduct, the entire surface can be electrically conductive, and forthese application pattern generally is not performed. For embodimentsinvolving patterning, the proportion of the surface comprising theelectrically conductive sintered network can generally be selected basedon the selected design. In some embodiments, the fused network comprisesfrom about 1 percent to about 99 percent of the surface, in furtherembodiments from about 5 percent to about 85 percent and in additionalembodiment from about 10 percent to about 70 percent of the substratesurface. A person of ordinary skill in the art will recognize thatadditional ranges of surface coverage within the explicit ranges aboveare contemplated and are within the present disclosure. The fusednetwork along the surface can form a conductive pattern with a singlepathway 21, as shown in FIG. 1A or with a plurality of electricallyconductive pathways 23, 25, and 27, as shown in FIG. 1B. As shown inFIG. 1B, the fused area forms three distinct electrically conductiveregions 23, 25, and 27. Although a single connected conductive regionand three independently connected conductive regions have beenillustrated in the figures, it is understood that patterns with two,four or more than 4 conductive independent conductive pathways orregions can be formed as desired. Similarly, the shapes of theparticular conductive regions can be selected as desired.

The difference between the electrical conductivity of the fused networkregions of the surface and the un-fused nanowire regions can providedesired functionality. In general, the variation in the electricalconductivity between the fused regions and the un-fused regions can bevery large, as described in the examples, although less large contrastscan still be effective. In general, the un-fused metal nanowire regionshave a sheet resistance that is at least about 10 times the sheetresistance of the fused metal network, in further embodiments at leastabout 100 times, in additional embodiments at least about 1000 times,and in other embodiments at least about 1,000,000 or greater times thesheet resistance of the fused metal network (e.g., up to at least 10⁹Ohms/sq or greater). High resistance measurements can be made, e.g., onunfused networks or bare polymer substrate, by first painting silverpaste onto the surface of the samples to define a square. The sample canthen be annealed at roughly 120° C. for 20 minutes in order to cure anddry the silver paste. Alligator clips are connected to the silver paste,and the leads can be connected to a suitable high resistance measurementdevice, such as an AlphaLabs High Resistance Low Conductance Meter underelectrical shielding. High resistance measurements was recorded and aredescribed in the examples below. The instrument can measure up to 1999gigaohm. A person of ordinary skill in the art will recognize thatadditional ranges within the explicit ranges above are contemplated andare within the present disclosure. The optical transparency to visiblelight can be approximately the same across the fused metal network andthe un-fused metal nanowire film. Thus, to the eye the film may lookuniform across the substrate surface in white light so that thepatterning looks invisible. An invisible pattern may be desirable forsome applications.

The patterning of fused and un-fused regions of the metal nanowire filmcan be driven by the selective delivery of the fusing agent. In general,the metal nanowire film can be first delivered to a surface. The metalnanowire film can be delivered to be relatively uniform across thesurface or some appropriate portion thereof. Of course, a fraction ofthe surface can remain uncoated at all with nanowire film, andreferences to patterning refer to the portions of the surface with thenanowire film, i.e. fused and un-fused portions of the film. Patterningwith halide based fusing agents is described in the '207 patent citedabove.

If a liquid solution comprising the fusing agent is applied to the metalnanowire film, the fusing solution can be delivered to the selectedportions of the film to perform the fusing. While a well-sealed mask canbe used to prevent contacting of the liquid fusing agent with selectedportions of the film, it can be desirable to print the liquid sinteringagent along the desired portion of the film using inkjet printing,screen printing or other appropriate printing process. The properties ofthe liquid fusing agent can be adjusted to be appropriate for theparticular printing approach. A small volume of liquid fusing agent canbe delivered to provide the appropriate fusing. The liquid and/or theprinting process can be controlled to limit the spreading of the fusingliquid or to have spreading controlled to provide fusing over a selectedregion. Moreover, conventional photolithography, such as usingphotoresist materials, can be used to make a mask to define regions inwhich the nanowires come in contact with the fusing solution.

Thermal fusing or sintering of silver nanowires can be difficult sincethe nanowires may not sinter until temperatures greater than thestability temperatures of polymer substrates. The use of radiation canbe used to address this difficulty by directing the radiation into thesilver nanowires while the polymer substrate may be relativelytransparent to the radiation so that the thermal load on the substrateis reduced. A report of laser ablation of silver nanowires is reportedin Pothoven, “Laser Patterning of Silver Nanowires,” Information DisplayMagazine, Vol. 28 (9), September 2012, electronic article, incorporatedherein by reference. While the laser ablation of the nanowires resultedin relatively modest damage to the substrate, the fusing of thenanowires can be performed with a significantly lower radiation dose,which should further reduce the radiation damage to the substrate formany substrates. While ultraviolet, visible and/or infrared light can beused to fuse the metal nanowires, infrared can be desirable due togenerally reduced damage to the substrate. A range of lasers areavailable with selected wavelengths, such as excimer lasers or YAGlasers, to pattern the radiation fusing of the film, and laser controlsystems can be used to scan the beam to selected locations. A commercialsystem directed to the scanned laser include, for example, a ScanLabsscanners. Also, high intensity infrared lamps, such as noble gas basedflash lamps, can be directed at the film to perform the sintering, and amask can be used to define the pattern. The use of high intensity heatlamps to perform a rapid thermal anneal on semiconductor devices isdescribed in U.S. Pat. No. 5,665,639 to Seppala et al., entitled“Process for Manufacturing a Semiconductor Device Bump Electrode Using aRapid Thermal Anneal,” incorporated herein by reference, and suchdevices can be adapted for metal nanowire fusing.

With respect to subtractive patterning, metal is removed from regionsidentified to have high electrical resistance. Removal of metal can beperformed effectively following fusing of the nanowires into a fusednetwork over the surface, although in some embodiments removal of metalnanowires prior to fusing or after patterned fusing can be performed. Ifremoval of metal nanowires for patterning is done prior to fusing, thefusing agent may then be applied over the whole patterned surface sincethe high resistance areas can be substantially free of metal nanowires.The removal of metal from regions in which a high electrical resistanceis desired can be performed through etching or with radiation. Toperform wet chemical etching, a patterned mask can be applied, forexample, using conventional photolithography and a photoresist. Suitablewet etching agents to remove metal nanowires can include, for example,8M nitric acid or 3M cupric chloride/hydrochloric acid. With a positivephotoresist, the applied photoresist is exposed and developed to exposeregions for etching, and following etching, the photoresist is removed.For radiation based removal of the metal, radiation can be usedsimilarly to the fusing of metal nanowires with radiation describedabove, except that an appropriately higher radiation dose is deliveredto remove the metal rather than fusing the metal. In particular, aninfrared laser or an infrared heat lamp can generally provide ablationfor the metal with reduced damage to the substrate relative to someablation approaches. Scanning of a laser beam or masking the radiationcan be used to define the patterning with the radiation. Subtractivepatterning can produce regions with surface resistances on the order of10⁷ to 10⁹ or greater ohms/sq.

The efficient patterning of the conductive transparent film can be veryeffective for certain display and or touch sensor applications. Inparticular, a touch sensor may desirably have patterns of electricallyconductive regions to provide for a corresponding pattern of touchsensors, and the transparency provides for the visualization of adisplay or the like under the pattern as shown below.

Touch Sensors

A common feature of the touch sensors generally is the presence of twotransparent conductive electrode structures in a spaced apartconfiguration in a natural state, i.e., when not being touched orotherwise externally contacted. For sensors operating on capacitance, adielectric layer is generally between the two electrode structures.Referring to FIG. 2A, a capacitance based touch sensor 101 comprises adisplay component 103, an optional bottom substrate 105, a firsttransparent conductive electrode structure 107, a dielectric layer 109,such as a polymer or glass sheet, a second transparent conductiveelectrode structure 111, optional top substrate 113, and measurementcircuit 115 that measures capacitance changes associated with touchingof the sensor. Referring to FIG. 2B, a resistance based touch sensor 131comprises a display component 133, an optional lower substrate 135, afirst transparent conductive electrode structure 137, a secondtransparent conductive electrode structure 139, support structures 141,143 that support the spaced apart configuration of the electrodestructures in their natural configuration, upper substrate 145 andresistance measuring circuit 147.

Display components 103, 133 can be LED based displays, LCD displays orother desired display components. Substrates 105, 113, 135, 145 can betransparent polymer sheets or other transparent sheets. Supportstructures can be formed from a dielectric material, and the sensorstructures can comprise additional supports to provide a desired stabledevice. Measurement circuits 115 and 147 are known in the art, and somespecific sensor embodiments are referenced below in the context ofpatterning. Transparent conductive electrodes 107, 111, 137 and 139 canbe effectively formed using sintered metal networks, although in someembodiments the sintered metal networks form some electrode structureswhile other electrode structures in the device can comprise materialssuch as indium tin oxide, aluminum doped zinc oxide or the like. Fusedmetal networks can be effectively patterned as described herein, and itcan be desirable for incorporate patterned films in one or more of theelectrode structures to form the sensors such that the plurality ofelectrodes in a transparent conductive structure can be used to provideposition information related to the touching process. The use ofpatterned transparent conductive electrodes for the formation ofpatterned touch sensors is described, for example, in U.S. Pat. No.8,031,180 to Miyamoto et al., entitled “Touch Sensor, Display With TouchSensor, and Method for Generating Position Data,” and published U.S.patent application 2012/0073947 to Sakata et al., entitled “Narrow FrameTouch Input Sheet, Manufacturing Method of Same, and Conductive SheetUsed in Narrow Frame Touch Input Sheet,” both of which are incorporatedherein by reference.

EXAMPLES

Commercial silver nanowires with different sizes were used in thefollowing examples. The properties of the silver nanowires had anaverage diameter of between 30 and 40 nm and an average length of 10-15microns. The silver nanowires (AgNWs) films were formed using thefollowing procedure. Commercially available silver nanowires (AgNWs)were dispersed in solvent to form an AgNWs dispersion. The AgNWsdispersions were typically in the 0.1-1.0% wt range in an alcoholsolvent. The dispersion was then deposited on glass or polyethyleneterephthalate (PET) surfaces as an AgNWs film using a spray coating or ahand-drawn rod approach. The AgNWs film was then exposed briefly toselective fusing agent to form fused metal nanostructured networks. Oneink or two ink systems were also developed that mixed AgNWs with fusingagents directly in ink solution.

The total transmission (TT) and haze of the AgNWs film samples weremeasured using a Haze Meter with films on a polymer substrate. The hazefrom the PET substrate alone was about 0.4%. To adjust the hazemeasurements for the samples below, a value of 0.4% can be subtractedfrom the measurements to get approximate haze measurements for thetransparent conductive films alone. The instrument is designed toevaluate optical properties based on ASTM D 1003 standard (“StandardTest Method for Haze and Luminous Transmittance of TransparentPlastics”), incorporated herein by reference. The total transmission andhaze of these films include PET substrate which has base totaltransmission and haze of ˜93.3% and 0.3%-0.4%, respectively. Sheetresistance was measured with a 4-point probe method unless indicatedotherwise. In the following examples, several different solutionsintering agents are presented. The transparent conductor propertiesbefore and after sintering (especially) and the sheet resistance of thefilms are provided. The network of nanowires can be composed of silverand some polymers which may serve as an ink dispersant or binder.Representative high resistance measurements were performed on theunfused metal nanowire films formed for these examples. To make themeasurements, a square of silver paste was painted onto the surface ofthe samples to define a square, which were then annealed at roughly 120°C. for 20 minutes in order to cure and dry the silver paste. Alligatorclips were connected to the silver paste, and the leads can be connectedto a suitable high resistance measurement device. Three representativemeasurements were >1000 gigaohm/sq. (10¹² ohm/sq.), 1000 gigaohm/sq and100 gigaohm/sq (10¹¹ ohm/sq).

Example 1—Fusing Compositions with Different Silver Sources, Acids andSolvents

This example test the fusing ability of different silver sourcescombined with several acids in different alcohols as fusing agents. Allacids used were at ˜0.01-1.0 μL/mL (0.001-1.0 Vol % i.e. Vol/Vol or0.00157M to 0.157M), and all silver salts used were at 1.0 mg/mL. Thecomposition of fusing agents 1-10 are listed in Table 2 below. Thefusing agents were applied to AgNWs films and the properties of thefilms before and after the fusing agent application are compared inTable 2, with films 4, 6, and 7 being control that comprise no acid orsilver salt. AgNWs inks were created in isopropyl alcohol or isobutylalcohol or other solvents at 0.01-0.5 wt %. The inks were then coatedusing a Meyer rod or blade coating. The films were quickly dried with aheat gun or IR lamp for a few seconds to flash off solvents. The fusingsolution was then deposited using spray or blade coating. After thecoating, the films were quickly dried using the heat gun or IR lamp todry the solvent. Equivalent silver nanowire films were used to testfusing solutions for Examples 1-9.

The resistances of the films 1-7 were measured and the results plottedin FIG. 3. As shown in FIG. 3, composition 1 appeared to cause no fusingas the resistance of the film remained high. Films treated withcompositions 2, 3, 4, 5, 6, and 7 all showed significantly reducedresistance, indicating fusing or sintering has occurred in these filmsamples. FIG. 3a is a composite of SEM images, taken at differentmagnifications, of the surface of a film representative of film sample 5and shows fusing or sintering of the nanowires. Fusing composition 7 inparticular contained only 1.0 ul/mL nitric acid in ethanol without anadded silver salt, and was able to sinter the nanowires successfully asindicated by the reduction in resistance. In particular, prior tosintering film 7 had a resistance of greater than 20,000 ohm/sq.Following sintering, film 7 had a resistance of about 150 ohm/sq. Sincesample 7 only has the silver nanowires as a metal source, this exampledemonstrated that an oxidizing acid, e.g., nitric acid, can mobilize themetal and induce fusing of adjacent nanowires into a fused network.

TABLE 2 Solution Ag⁺ source Sinter 1 HF in EtOH NONE NO 2 AgNO₃ in HF(EtOH) Yes (NO₃) YES 3 AgF HF (EtOH) Yes (F) YES 4 AgNO₃ in IPA Yes(NO₃) YES 5 AgF & HNO₃ in EtOH/H₂O Yes (F) YES (80:20 v:v) 6 AgNO₃ + AgFin EtOH Yes (NO₃) and YES (F) 7 HNO₃ in EtOH NONE YES

Example 2 Nitric Acid in Ethanol as Fusing Solution

The sintering ability of nitric acid in ethanol as fusing agent isfurther tested in this example. Specifically, solutions containing˜0.01-1.0 uL/mL nitric acid in EtOH were cast by blade coating onto dryAgNW films that were formed by blade coating AgNW dispersions onto thesubstrate. Four replicate film samples were formed. The resistance ofthe films before and after the application of the sintering solution wasmeasured and the results plotted in FIG. 4. The variation in resistancewas likely due to variations in the films or the correspondingdispersions, and/or the coating quality. As shown in FIG. 4, the initialresistances of the films were as high as 20,000 ohm/sq while thecorresponding sintering solution treated films showed resistance to theright of between 80-140 ohm/sq, with >90 Total Transmission (TT)(including the PET substrate) and low haze. Since the PET substrate hasa TT of roughly 93%, the TT of the conductive film was correspondinglyhigher. It should be noted that initial resistance of these samplesprior to the treatment may be much greater than the recorded 20,000ohm/sq but is out of the detection range for the hand held four-pointprobe used. Significant reduction in resistance in the films thereforewas observed accompanied by retention of high TT and low haze.

Example 3 Silver Nitrate and Nitric Acid in Ethanol as Fusing Solution

The sintering ability of silver nitrate and nitric acid in ethanol asfusing agent is tested in this example. AgNW films 1-6 were cast from a0.01-0.5 wt % solution. Fusing solution containing 0.1 mg/ml AgNO₃ and1.6 uL/mL HNO₃ in EtOH were applied to the films. The resistance, totaltransmission and Haze include PET of the films were measured and theresults listed in Table 3 below. As shown in Table 3, the initialresistances of the networks of AgNW films 1-6 were shown to be higherthan 20,000 ohm/sq. After treatment with the fusing solution, the AgNWfilms 1-6 were shown to have resistance between 75-203 ohm/sq, with >90Total Transmission (TT) (including the PET substrate) and low haze.Since the PET substrate has a TT of roughly 93%, the TT of theconductive film was correspondingly higher. Significant reduction inresistance in the films therefore was observed accompanied by retentionof high TT and low haze. Relative to corresponding film samples formedwithout AgNO₃ (not shown), the results presented in Table 3 weregenerally more consistent. FIG. 4a is a composite of SEM images ofdifferent portions of a film representative of films 1 and 2 in Table 3.The SEM images demonstrate sintering of AgNW.

TABLE 3 Before After Final TT % (ohm/sq) (ohm/sq) (%) Haze 1 >20,000 20391.4 1.11 2 >20,000 203 92.1 0.98 3 >20,000 75 >90.0 1.51 4 >20,000174 >90.0 1.24 5 >20,000 100 >90.0 1.11 6 >20,000 111 92.9 0.83

Example 4 Fusing Compositions with Different Silver Sources in Ethanol

This example tests the fusing ability of different silver sourcescombined with nitric acid or perchloric acid in ethanol. All silversources were 0.1 mg/ml and all acids were 1.6 uL/mL in EtOH. The silversources used include AgF, Ag-Acetate, Ag-trifluoromethane sulfonate(AgTMS), Ag-Heptafluorobutyrate (AgFHB), and Ag-Hexafluoroantimonate(AgHFA). The fusing reagents used, the resistance, total transmissionand haze include PET of the films were measured and the results listedin Tables 4 and 5 below. As shown in Tables 4 and 5, the initialresistances of the AgNW films were shown to be higher than 20,000ohm/sq. After treatment with the fusing agents, the AgNW films wereshown to have resistance between 50-200 ohm/sq with the exception ofAgBF₄ treated film which as a resistance of 500 ohm/sq. The totaltransmission of (TT) films after the treatment appear to be higher than91 with low haze with the exception of perchloric acid—AgHClO₄ treatedfilm which has a haze of 11.4. Significant reduction in resistance inthe most of the films therefore was observed accompanied by retention ofhigh TT and low haze.

TABLE 4 Silver ohm/sq ohm/sq Final Final Source Acid Before After TTHaze AgF HNO₃ >20,000 110 91.7 1.14 Ag-Acetate HNO₃ >20,000 150 91.51.33 Ag-TMS HNO₃ >20,000 75 91.8 0.93 AgFHB HNO₃ >20,000 90 91.5 0.96AgHFA HNO₃ >20,000 110 91.1 1.07 AgBF₄ HNO₃ >20,000 500 91.7 1.5 AgHClO₄HClO₄ >20,000 200 92.5 11.4

TABLE 5 Silver ohm/sq ohm/sq Final Final Source Acid Before After TTHaze AgHFB HNO₃ >20,000 103 92.4 0.88 AgHFB HNO₃ >20,000 108 92.5 1.05AgHFB HNO₃ >20,000 50 92.1 1.24 AgTMS HNO₃ >20,000 95 92.0 1.03 AgTMSHNO₃ >20,000 100 92.2 0.91 AgTMS HNO₃ >20,000 103 92.2 0.92

Example 5 Comparison of Silver Nitrate and Silver Fluoride in Ethanol asFusing Agents

This example compares the fusing ability of silver nitrate and silverfluoride when combined with nitric acid in ethanol as fusing agents ondifferent AgNW films. AgNW films were cast from a 0.01-0.5 wt %solution. Fusing agent used was 0.1 mg/ml AgF or AgNO₃ and 1.6 uL/mLHNO₃ in EtOH. Four replicate film samples were made using AgF as afusing agent and four replicate samples were made using AgNO₃ as afusing agent. The sintering reagents used in each film sample and thecorresponding resistance, total transmission and haze include PET of thefilms were measured and the results listed in Table 6 below. Thedifferences between like film samples were likely due to variations incoating or the quality of the AgNW dispersion prior to coating. As shownin Table 6, the initial resistances of the AgNW films were shown to behigher than 20,000 ohm/sq. After treatment with the fusing agents, theAgNW films were shown to have resistance less than 150 ohm/sq for AgFtreated films and less than 100 ohm/sq for AgNO₃ treated films. Thetotal transmission of (TT) films after the treatment appears to behigher than 91% for AgF treated films and higher than 92% for AgNO₃treated films with low haze of less than 1.4% for AgF treated films andless than 1.02% for AgNO₃ treated films. Although significant reductionin resistance in the films therefore was observed accompanied byretention of high TT and low haze were observed in both silver nitrateand silver fluoride, silver nitrate appeared to produce films with lowerresistance, higher TT and lower haze.

TABLE 6 Silver ohm/sq ohm/sq Final Final Source Acid Before After TTHaze AgF HNO₃ >20,000 <150 91.2 1.35 AgF HNO₃ >20,000 <150 91.7 1.16 AgFHNO₃ >20,000 <150 91.4 1.26 AgF HNO₃ >20,000 <150 91.5 1.37 AgNO₃HNO₃ >20,000 <100 92.5 1.00 AgNO₃ HNO₃ >20,000 <100 92.5 1.02 AgNO₃HNO₃ >20,000 <100 92.5 0.88 AgNO₃ HNO₃ >20,000 <100 92.5 0.99

Example 6 Palladium Sintering Solutions

This example demonstrates the ability to fuse metal nanowires with ametal element different from the metal of the nanowires. Palladiumsintering solutions therefore were tested as fusing agent in thisexample.

Palladium (Pd) salts were made 0.0005-0.005 molar in Ethanol. HNO₃ wasadded 1.0 uL/mL. The fusing reagents used in each film sample and thecorresponding resistance and total transmission including PET of thefilms were measured and the results listed in Table 7 below. Since thePET substrate has a TT of roughly 93%, the TT of the conductive film wascorrespondingly higher. As shown in Table 7, the initial resistances ofthe AgNW films were shown to be higher than 20,000 ohm/sq. Aftertreatment with the fusing agents, although the resistance of the filmshas been significantly decreased, it is still high compared to after thetreatment by other fusing agents. Only Pd(NO₃)₂ with nitric acid treatedfilm was shown to have resistance less than 150 ohm/sq. Using Pd(NO₃)₂with nitric acid as fusing agent has been explored further as describedbelow in Example 6.

TABLE 7 Palladium ohm/sq ohm/sq Final Source Acid Before After TTK₂PdCl₄ NONE >20,000 1,130 86.8 K₂PdCl₄ HNO₃ >20,000 880 85.9 Pd(NO₃)₂NONE >20,000 235 90.2 Pd(NO₃)₂ HNO₃ >20,000 140 91.1

Example 7 Other Metal Sintering Solutions

Metal sources other than silver such as palladium and copper were usedin this example as fusing agents. Palladium (Pd) or copper salts weremade 0.00005-0.005 molar in ethanol. HNO₃ was added 1.6 μL/mL. Forfusing solutions Pd2 and Pd4, 0.1 mg/mL AgF was added additionally toform the sinter solution. The fusing agents used in each film and thecorresponding resistance, total transmission and haze include PET of thefilms were measured and the results listed in Table 8 below. The initialresistances of the AgNW films were higher than 20,000 ohm/sq. Theresistance after the treatment was an average obtained from three datapoints. As shown in Table 8, after treatment with the fusing agents, theAgNW films were shown to have resistance between 32 and 117 ohm/sq. Thetotal transmission of (TT) films after the treatment appears to bebetween 88 and 91 including the PET substrate. The Veeco AFM instrumentwas used and 10×10 μm scans were captured of the AgNW network formedwith the sample Pd3 was conducted and shown in FIG. 5, indicating cleargrowth of the nanowire network. FIG. 5a is a composite of SEM images,taken at different magnifications, of the surface of a filmrepresentative of sample Pd1 in table 8 and shows fusing or sintering ofthe nanowires.

TABLE 8 Sinter Resistance % Solution (Ohm/sq) TT Pd1 5 × 10⁻⁴ Pd(NO₃)₂ +1.6 ul/ml HNO₃ 41 89.7 Pd2* 5 × 10⁻⁴ Pd(NO₃)₂ + 1.6 ul/ml HNO₃ + AgF 3288.7 Pd3 5 × 10⁻⁵ Pd(NO₃)₂ + 1.6 ul/ml HNO₃ 117 90.6 Pd4* 5 × 10⁻⁵Pd(NO₃)₂ + 1.6 ul/ml HNO₃ + AgF 33 90.3 Cu1 5 × 10⁻³ Cu(NO₃)₂ + 1.6ul/ml HNO₃ 65 88.7

Example 8 Comparison of Copper and Silver Salts

Copper is used as the metal salt in the sintering solution and comparedto silver salt in this example. The fusing reagents used in each filmsample and the corresponding resistance, total transmission and hazeinclude PET of the films were measured and the results listed in Table 9below. The initial resistances of the AgNW films were higher than 20,000ohm/sq. The resistance after the treatment was an average obtained fromthree data points. It took about 10 mins to form the sintered networkusing the copper solution comprising ethanol.

TABLE 9 Metal Sinter Ohm/ % Used Solution sq TT Haze Cu1a Copper 5 ×10⁻⁴ molar Cu(NO₃)₂ + 78 90.1 1.63 1.6 μl/ml HNO₃ Control Silver 0.1mg/ml AgNO₃, 63 90.8 1.58 1.6 μl/ml HNO₃

Copper solutions showed similar performance as silver based (AgNO₃, HNO₃system). Better haze and optics for Cu compared to previous experimentwas obtained by reducing the concentration of copper used by tenfold.The AFM of the AgNW network formed with the diluted copper solution(5×10⁻⁴ Cu(NO₃)₂) was conducted and shown in FIG. 6, indicating cleargrowth of the nanowire network.

Example 9 Gold and Lead Used in Fusing Solutions

Gold and lead were used as the metal salt in the fusing solution in thisexample. The fusing reagents used in each film sample and thecorresponding resistance, total transmission and haze include PET of thefilms were measured and the results listed in Table 10 below. Theinitial resistances of the AgNW films were higher than 20,000 ohm/sq.The resistance after the treatment was an average obtained from threedata points. The fusing solution containing gold appeared to lift theAgNW off the base PET. No resistance was obtained for the film aftertreatment with gold containing fusing solution. This is believed to bedue to poor adhesion of the resulting film, and not an inherent propertyof the gold based fusing solutions. The resistance of the film afterlead treatment appears to be relatively high, about 316 ohm/sq. The leadtreated film appeared to have good TT and low haze.

TABLE 10 Sinter % Metal Solution Resistance TT Haze Au1 Gold 1 × 10⁻³molar HAuCl₄ + n.a. 89.8 0.87 (10 μl HNO₃/ml Ethanol) — Pb1 Lead 1 ×10⁻³ molar Pb* 316 90.2 1.21 Acetate + HNO₃

Example 10 One Ink Solution Fusing

This experiment explores the process of mixing fusing agents and AgNWink together in solution and coat out on to a base substrate such asPET. Several organic and alcohol based solvents were used. The one inkin general refers to systems where the fusing agents have beenintegrated into the ink. The reducing agent used in these embodiments isgenerally the solvent, or a reducing agent can be added into the fusingsolution. A summary of the various kinds of chemicals and mechanismsused for fusing has been provided. Many volume/concentration ratios werescreened, in some cases small volumes of fusing solution (FS) was addedinto the AgNW ink; in other cases AgNWs was added directly to fusingsolution (FS). Three sintering solutions (FS) were explored: CT1) AgF(0.1 mg/ml)+HNO₃ (1.6 uL/ml); CT2) AgNO₃ (0.1 mg/ml)+HNO₃ (1.6 uL/ml);and CT3) Plated out AgF (1.0 mg/ml in EtOH which was bath sonicated for30 mins and then filtered through either 0.45 or 0.2 um PTFE filters).Because HNO₃ can etch AgNW, it is therefore important to make solutionsCT1 and CT2 right before the coating process. AgNW dispersion wereprepared that has a concentration of 0.01-0.5 weight percent in ethanolor isopropyl alcohol. The fusing solutions were also prepared in theethanol in specified concentrations. Isopropyl alcohol (IPA) is used asa diluting solvent. The ratio of AgNW/IPA/FS in mL are specified intables 10-12 below. The resistance of the films after the treatment wasan average obtained from three data points.

AgF+HNO₃ (CT1) Dilutions into AgNW Ink

Fusing solution (FS) CT1 was added to the AgNW suspension to replacepartially or all of the IPA diluents as specified in the Table 11 below.The first sample contained no FS and HCl vapor was used as the fusingagent to serve as a control. As shown by the performance data in Table11, when concentration of the FS is equivalent to over-coatconcentrations (i.e. the concentrations used to coat the fusing solutionon top of an unfused network), similar resistance, TT, and haze valuewas achieved in sample 5 as compared to the control sample 1. Addingsmall amounts of sintering agents contained in CT1 therefore does appearto fuse the AgNWs to form the nanostructured network, as evidenced bythe improvement in the resistance of the AgNW films. The removal of theIPA diluent resulted in results similar to the HCl vapor control. FIG.6a is a composite of SEM images, taken at different magnifications, ofthe surface of a film representative of film sample 1 and shows fusingor sintering of the nanowires.

TABLE 11 AgNW/IPA/FS Ohm/ % TT/ (ml) Ink Composition sq Haze 1.0.2/0.9/0 Dilution Control 99 91.0/1.17 and HCl Vapor 2. 0.2/0.81/0.09Reduce IPA 10% and — 91.6/1.36 add 10% CT1 3. 0.2/0.675/0.225 Reduce IPA25% and 2500 91.3/1.43 add 20% CT1 4. 0.2/0.450/0.450 Reduce IPA 50% and694 91.6/1.42 add 50% CT1 5. 0.2/0/0.9 Reduce IPA 100% and 137 91.5/1.21use AgNW in CT1

AgNO₃+HNO₃ (CT2) Dilutions into AgNW Ink

Fusing solution (FS) CT2 was added to the AgNW suspension to replacepartially or all of the IPA diluents as specified in the Table 12 below.The first sample contained no FS and HCl vapor was used as the sinteringagent to serve as a control. As shown by the performance data in Table12, when concentration of the FS is equivalent to over-coatconcentrations, similar resistance, TT, and haze value was achieved insample 5 as compared to the control sample 1. Adding small amounts ofsintering agents contained in CT2 therefore does appear to fuse theAgNWs to form the nanostructured network, as evidenced by theimprovement in the resistance of the AgNW films. This approach couldpotentially significantly lower the haze of the treated film as only onecoat was coated onto the substrate compared to the film with anadditional over coat of sintering solution. For these solutions, removalof all of the IPA diluent resulted in a lower sheet resistance and alower haze relative to the HCl vapor control with a small decrease in %T. FIG. 6b is a composite of SEM images, taken at differentmagnifications, of the surface of a film representative of the filmsformed with CT2 in Table 12, however, having a sheet resistance of about200 Ohm/sq. FIG. 6b demonstrates significant sintering or fusing of thenanowires.

TABLE 12 AgNW/IPA/FS Ohm/ % TT/ (ml) Ink Composition sq Haze 1.0.2/0.9/0 Dilution Control 115 91.6/1.69 and HCl Vapor 2. 0.2/0.81/0.09Reduce IPA 10% and — 91.6/1.21 add 10% CT2 3. 0.2/0.675/0.225 Reduce IPA25% and — 91.4/1.24 add 20% CT2 4. 0.2/0.450/0.450 Reduce IPA 50% and6603 91.4/1.33 add 50% CT2 5. 0.2/0/0.9 Reduce IPA 100% and 86 90.9/1.40use AgNW in CT2

Filtered AgF (CT3)

Fusing solution (FS) CT3 containing only 0.01-0.1 weight percent AgFwithout the acid was added to the AgNW suspension to replace partiallyor all of the IPA diluents as specified in the Table 13 below. The firstsample contained no FS and HCl vapor was used as the fusing agent toserve as a control. As shown by the performance data in Table 13, whenconcentration of the FS is equivalent to over-coat concentrations,similar resistance, TT, and haze value was achieved in sample 5 ascompared to the control sample 1. Adding small amounts of fusing agentscontained in CT3 therefore does appear to fuse the AgNWs to form thenanostructured network, as evidenced by the improvement in theresistance of the AgNW films. At lower CT3 ratios however, the fusingdid not occur after one hour of coating. The low resistance observed insample 5 was after coating the substrate with the mixed solution after 1hour. It should be noted that without additional acid in the system, AgFplated out 30 mins after coating.

TABLE 13 AgNW/IPA/FS Ohm/ % TT/ (ml) Ink Composition sq Haze 1 0.2/0.9/0Dilution Control 109 91.2/1.16 and HCl Vapor 2 0.2/0.81/0.09 Reduce IPA10% and — 91.1/1.42 add 10% CT3 3 0.2/0.675/0.225 Reduce IPA 25% and —91.4/1.34 add 20% CT3 4 0.2/0.450/0.450 Reduce IPA 50% and — 92.1/1.03add 50% CT3 5 0.2/0/0.9 Reduce IPA 100% and  69 91.6/1.16 use AgNW inCT3

Example 11 Two Ink Solution Fusing

A two ink solution sintering system has been developed and shown in thisexample. A two ink solution sintering system is one in which an AgNW inkis used to form a first coating and a distinct FS ink is used to form asecond coating on top of the AgNW ink coating. Specifically, propyleneglycol (PG) has been added to isobutanol (IBA) with PG serving as thereducing agent. Compared to the EtOH or IPA systems, IBA in the two inksystem has a higher boiling point and higher viscosity, which isbelieved to lead to more uniform coating. However, unlike EtOH, IBA doesnot reduce the Ag⁺ as readily so PG is added to augment the reducingcapability of this system. The ink had an AgNW concentration of 0.01-0.5weight percent. All films were made on a PET sheet, which had a haze of˜0.3-0.4, and a % TT of ˜93.0-93.3 including the PET substrate.

Solutions 1-4 listed in the Table 14 below were made with the mixturesolvents of IBA and PG in a 90:10 ratio. On average 9 resistance datapoints were collected and the resistance data shown in Table 14 is anaverage of these 9 data points. The films without the fusing treatmenthad resistance >20,000 ohm/sq. As shown in Table 14 below, upon fusingtreatment, significant reduction in resistance were observed for allsamples with high TT and low haze.

TABLE 14 % TT/ Solution # Sintering Solution Ohm/sq Haze 1 1.6 uL/mLHNO₃ + 110 91.1/1.24 0.1 mg/ml AgNO₃ 2 1.6 uL/mL HNO₃ + 145 91.7/1.070.1 mg/ml AgF 3 0.16 uL/mL HNO₃ + 181 >90.0 0.1 mg/ml AgNO₃ 4 0.16 uL/mLHNO₃ + 69 90.8/1.46 0.1 mg/ml AgF

Example 12 IBA Solution Ink with Organic Reducing Agents

One-ink systems were created by adding various reducing agents at smallquantities into the IBA ink. Table 15 provides a list of reducing agentsthat has shown to provide fusing in the one ink system. These reducingagents vary in terms of reduction strength and can also have otherinteresting properties. For example, during oxidation the reducingagents can change color which may improve the haze of the overall film.

TABLE 15 Chemical Name Chemical Formula Fuse Chemical StructureAminophenol: H₂NC₆H₄OH Molecular Weight 109.13 YES

Hydroquinone C₆H₄-1,4-(OH)₂ Molecular Weight 110.11 YES

Benzoin: C₆H₅CH(OH)COC₆H₅ Molecular Weight 212.24 YES

EBPB: CH₃CH{C₆H₂[C(CH₃)₃]₂OH}₂ Molecular Weight 438.69 YES

4-Amino-3-hydroxy- 1- naphthalenesulphonic acid H₂NC₁₀H₅(OH)SO₃HMolecular Weight 239.25 YES

Sample solutions 1-6 listed in the table 16 below were made with IBA andvarious reducing agents. On average 6 to 9 resistance data points werecollected and the resistance data shown in Table 16 is an average ofthese 6 to 9 data points. The films without the sintering treatment wereformed from AgNW inks comprises 0.01-0.5 weight percent AgNW in IBA andthe films had resistance >20,000 ohm/sq. Fused films were formed bycoating a thin layer (about 50 μm wet thickness) of the FS over adeposited AgNW film using blade coating and then dried. As shown inTable 15 below, upon fusing treatment, significant reduction inresistance were observed for samples contained benzoin and aminophenol.With added reducing agent, other higher boiling point solvents may alsobe used.

In general, the films after fusing treatment showed high TT and lowhaze. During drying the Ag⁺ and reducing agent were concentrated and theAg⁺ was reduced to metallic Ag^(o). Based on the results with the CT3reducing agent in Example 10, the AgF nitric acid solutions would not beexpected to be effective alone to significantly fuse the nanowireswithout the reducing agent even though the fluoride ion can inducesintering of the nanowires.

TABLE 16 Sample Solutions in Isobutanol Ohm/sq % TT 1 0.1 mg/ml AgF, 1.6uL/mL HNO₃, 62 90.7 0.17 mg/ml Benzoin 2 0.5 mg/ml AgF, 8 uL/mL HNO₃,288 >90.0 0.85 mg/ml Benzoin 3 0.5 mg/ml AgF, 8 uL/mL HNO₃, 144 >90.00.17 mg/ml Benzoin 4 0.5 mg/ml AgF, 8 uL/mL HNO₃, 179 >90.0 0.35 mg/mlEPBP 5 0.5 mg/ml AgF, 8 uL/mL HNO₃, 457 >90.0 0.20 mg/ml AHNS 6 0.5mg/ml AgF, 8 uL/mL HNO₃, 138 92.3 0.1 mg/ml Aminophenol

The AFM of the film with benzoin based solution sintering (sample 1) wasplotted in FIG. 6. As show in FIG. 7, growth is preferential at roughareas with areas where interaction with seeding nucleus is strongest andbarrier to formation is least. Growth appears suppressed on PET whichmakes sense since the surface energy of the AgNWs is much higher thanPET.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein.

What is claimed is:
 1. A method for forming a fused metal nanostructurednetwork, the method comprising contacting metal nanowires with a fusingsolution comprising a reducing agent source and a metal ion source toreduce metal ions to its corresponding metal element to fuse the metalnanowires together to form the fused metal nanostructured network. 2.The method of claim 1 wherein the metal ions from the metal ion sourcecomprises metal elements that are the same as the metal element of themetal nanowires.
 3. The method of claim 1 wherein the metal ions sourcecomprises metal ions from a dissolved salt and wherein the metal ionscomprises metal elements that are the different from the metal elementof the metal nanowires.
 4. The method of claim 1 further comprisingconcentrating the fusing solution by removing at least partially thesolvent of the fusing solution to effect the reduction and fusing of themetal nanowires.
 5. The method of claim 1 wherein the fusing solutioncomprises an acid.
 6. The method of claim 5 wherein the fusing solutionis free of added metal compositions and the acid is effective to ionizea portion of the metal nanowires as a metal ion source.
 7. The method ofclaim 1 wherein the contacting step comprises forming a metal nanowirefilm on a substrate and applying the fusing solution to the metalnanowire film to form the fused metal nanostructured network.
 8. Themethod of claim 7 wherein the fusing solution is applied over only aportion of the metal nanowire film to form a patterned fused metalnanostructured network on the substrate, wherein the contacted portionhas a sheeting resistance at least about 10 times less than the sheetresistance of the portion that is not contacted by the fusing solution.9. The method of claim 1 wherein the fusing solution further comprisesmetal nanowires and fusing solution and the method further comprisingdepositing the fusing solution on a substrate to form a film and atleast partially drying the film to form the fused metal nanostructurednetwork.
 10. The method of claim 1 wherein the metal nanowires comprisesilver.
 11. The method of claim 10 wherein the metal ion sourcecomprises metal ions from a dissolved salt wherein the metal ionscomprise silver ions, copper ions, palladium ions, gold ions, tin ions,iron ions, cobalt ions, zinc ions, aluminum ions, platinum ions, nickelions, cobalt ions, titanium ions, or combinations thereof.
 12. Themethod of claim 1 wherein the fused metal nanostructured network issupported on a substrate to form a film on a surface of the substrate.13. The method of claim 12 wherein the film has a sheet resistance of nomore than about 300 ohm/sq, and a total transmission of visible light ofat least about 91%.
 14. The method of claim 12 wherein the film has ahaze of no more than 0.8%.
 15. The method of claim 12 wherein the filmhas a sheet resistance of no more than about 100 ohm/sq.
 16. The methodof claim 12 wherein the fused metal nanostructured network has a loadingdensity on the substrate from about 0.01 mg/m2 to about 200 mg/m2. 17.The method of claim 12 further comprising rinsing and drying the film.18. The method of claim 12 further comprising removing a portion of thefused metal nanostructured network along a selected pattern to form apatterned fused nanostructured network.
 19. The method of claim 18wherein the removing of a portion of the fused metal nanostructurednetwork comprises applying an etching solution corresponding with theselected pattern.
 20. The method of claim 18 wherein the removing of aportion of the fused metal nanostructured network comprises ablatingwith radiation to removing a portion of the fused metal nanostructurednetwork to form the patterned fused metal nanowire network.
 21. Themethod of claim 1 further comprising prior to contacting the fusingsolution with the metal nanowires, depositing a film of metal nanowiresonto a substrate surface and ablating a portion of the film of metalnanowires, wherein a patterned fused nanostructured network is formedfollowing the contacting step.
 22. A method for forming a fused metalnanostructured network, the method comprising irradiating a metalnanowire film on a substrate surface to form the fused metalnanostructured network.
 23. The method of claim 22 wherein theirradiation is performed over only a portion of the metal nanowire filmto form a pattern of fused metal nanostructured network having a sheetresistance of no more than about 300 ohms/sq.
 24. The method of claim 23wherein the irradiation is performed through a mask with an infraredlamp.
 25. The method of claim 23 wherein the patterning is performed byscanning a laser beam to selected portions of the metal nanowire film toperform the pattern formation.
 26. A patterned transparent conductivematerial comprising a substrate, a fused metal nanostructured networkcovering a portion of a surface of the substrate, and regions of thesurface of the substrate substantially free of metal nanowires and fusedmetal networks, wherein the fused metal nanostructured network forms anelectrically conductive pattern and the transparent conductive materialhas a total transmission of visible light of at least about 91%.
 27. Thepatterned transparent conductive material of claim 26 havingapproximately uniform transparency across the surface.
 28. The patternedtransparent conductive material of claim 26 wherein the regions of thesurface of the substrate substantially free of metal nanowires have asheet resistance at least about 1000 times the sheet resistance of thefused metal network.
 29. A touch sensor comprising a first electrodestructure and a second electrode structure spaced apart in a naturalconfiguration from the first electrode structure, the first electrodestructure comprising a first transparent conductive electrode on a firstsubstrate wherein the first transparent conductive electrode comprises apatterned transparent conductive material of claim
 26. 30. The touchsensor of claim 29 wherein the second electrode structure comprises asecond transparent conductive electrode on a second substrate whereinthe second transparent conductive electrode comprises a second patternedtransparent conductive material of claim
 44. 31. The touch sensor ofclaim 29 wherein the first electrode structure and the second electrodestructure are spaced apart by a dielectric layer and further comprisinga circuit connected to the conductive electrode structures that measureschanges in capacitance.
 32. The touch sensor of claim 29 furthercomprising display components associated with the substrate.